The --remove-short-arcs option is no longer needed

These were not caused by bad map data, or something that you could do
anything about. The solution was to give the
--remove-short-arcs option. If you did not give that
option, then you could get routing errors in addition to those error
messages. This meant that the option was effectively required.

This has now been fixed and in the latest versions of mkgmap the
option is no longer required and at some point in the future it will
be removed altogether.